In recent years, attention has been focused on power storage systems for small, high energy density uses such as information-related equipment and communication equipment, and more specifically, personal computers, video cameras, digital still cameras, mobile phones and the like, and power storage systems for large, power uses such as auxiliary power sources for electric vehicles, hybrid vehicles and fuel cell vehicles, power facilities and the like.
As candidates for the power storage systems, there have been increasingly developed non-aqueous electrolyte lithium batteries including lithium ion batteries, lithium batteries and lithium ion capacitors. In the non-aqueous electrolyte lithium batteries, non-aqueous electrolytes, each prepared by dissolving a fluorine-containing electrolyte compound e.g. LiPF6 in a solvent e.g. cyclic carbonate, chain carbonate or ether, are commonly used for high battery voltage and capacity. However, the non-aqueous electrolyte lithium batteries using those non-aqueous electrolytes do not always achieve satisfactory battery characteristics such as cycle characteristics and output characteristics.
The non-aqueous electrolyte lithium batteries currently in practical use have the possibility of significant deterioration in battery characteristics, e.g. extremely shortening of battery lifetime, due to electrolyte decomposition at electrode surfaces during charge/discharge cycles under an environment temperature exceeding 60° C. In particular, the batteries for auxiliary power sources for electric vehicles, hybrid vehicles and fuel cell vehicles, home power facilities etc. cause large heat generation during charge/discharge cycles because of their high capacity and output performance. Further, these batteries are used outdoors so that the environment temperature of the batteries tend to be high in summer. Thus, cooling mechanisms are provided to keep the environment temperature of the batteries at 60° C. or lower. However, the cooling mechanisms are also operated by energy from the batteries. There has accordingly been a demand to reduce the energy consumption of the cooling mechanisms, or to develop electrolytes for non-aqueous electrolyte lithium batteries usable at an environment temperature of higher than 60° C. so as to show less deterioration in battery characteristics during charge/discharge cycles under a high temperature environment of e.g. about 80° C. and thereby eliminate the need to use cooling mechanisms.
Patent Document 1 discloses that, in the case of using a non-aqueous electrolyte in which a fluorine-containing electrolyte compound e.g. LiPF6 is dissolved in a solvent with the addition of lithium difluorophosphate, there occurs reaction of lithium difluorophosphate with electrode surfaces during initial charge/discharge cycles such that good coating films are formed on positive and negative electrodes so as to suppress reaction of the electrolyte solvent after the formation of the coating films and maintain battery discharge capacity after the storage for 20 days at 60° C. Patent Document 2 discloses that, in the case of using a LiPF6-containing electrolyte with the addition of a difluorophosphate salt, the battery shows improved output performance even after repeated charge/discharge cycles under an environment of 60° C. The addition of lithium difluorophosphate is in fact effective in improving battery cycle characteristics, but does not succeed in achieving sufficient battery cycle characteristics under a high temperature environment of about 80° C.
Further, Non-Patent Document 1 discloses that, in the case of using LiPF6 as an electrolyte salt in a lithium ion battery (lithium secondary battery), LiPF6 is decomposed by moisture absorption to form acidic components such as HF, POF3, H(OPOF2), H2(O2POF) and H3(PO4) that adversely affect battery characteristics.